The invention relates to a color television display tube comprising an envelope having a substantially rectangular display window and a substantially rectangular color selection electrode which is suspended in the corners of the display window with the aid of suspension means comprising flat resilient elements.
Such a color television display tube is disclosed in U.S. Pat. No. 3,548,235. This patent describes means for the suspension of a color selection electrode, and in particular for a shadow mask which does not have a rigid supporting frame connected thereto. The shadow mask is formed from a substantially rectangular, comparatively flexible, thin metal sheet having a large number of apertures. A mask ring of substantially the same thickness as the mask sheet is connected to the edge of the sheet. The mask ring gives the shadow mask a certain rigidity along the sides, but the shadow mask can be comparatively easily twisted about the diagonals and consequently has four hinge points at the corners. The shadow mask is suspended in the four corners of the display window with the aid of suspension means which have flat resilient elements which permit expansion of the shadow mask when the temperature increases. The suspension means further comprise clamping springs which are secured in chamber-like recesses in the corners of the display window. The resilient elements are connected to the clamping springs at their free ends.
In such a suspension of the shadow mask the desired distance between the shadow mask and the display window is determined by all four suspension points in the corners of the shadow mask rather than by only three, because the shadow mask can be twisted about a diagonal. The position of the shadow mask relative to the display window in directions parallel to the display window is established by three of these four suspension points. The position of the fourth suspension point is determined by the other three points. The fourth suspension point should assume an equilibrium position relative to the other three points, but it is determined by the position of the clamping spring in the chamber-like recess. As a result of this moments are exerted on the shadow mask by the suspension means so that the shadow mask is deformed. In addition, forces which occur during shocks or vibrations of the display tube are transmitted largely to the shadow mask, causing fading in the displayed picture.
U.S. Pat. No. 3,999,098 discloses a suspension of a shadow mask in the corners of a display window in which the parts of the suspension means connected to the shadow mask are provided with very rigid leaf springs which extend substantially parallel to the tube axis. Three of the four leaf springs are each provided with an aperture. The fourth leaf spring includes a tapering mandril. The part of the suspension means connected to the display window is formed by four metal supports sealed in the corners of the display window and extending substantially perpendicularly to the diagonals of the display window. Three of the four supports are provided with respective tapering mandrils. The fourth support has a substantially rectangular slot. The leaf springs having apertures fall over the mandrils of the supports and the mandril of the fourth leaf spring falls in the rectangular slot of the fourth support. The fourth suspension point together with the three other suspension points establish the distance between the shadow mask and the display window. The position of the shadow mask in directions parallel to the display window is established by three identical suspension points in the corners of the shadow mask. Since the mandril of the leaf spring at the fourth suspension point is provided in the rectangular slot of the fourth support, the fourth suspension point can seek an equilibrium position.
The natural rigidity of the shadow mask in such a suspension system is relied on to withstand the forces which the very rigid leaf springs exert. In addition, the position of the shadow mask is not unambiguously established because the position of the fourth suspension point is not fixed and poor assembly reproducibility is obtained during repeated attachment and removal of the shadow mask to and from the display window. This repeated attachment and removal of the shadow mask is necessary when the display screen is photographically provided on the window. Assembly reproducibility is to be understood to mean herein the extend to which the shadow mask resumes the same position each time it is reattached.